Universal Serial Bus, or
USB, is a serial standard to interface devices.
USB was designed to allow many peripherals to be connected using a single standardized interface and to improve the plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot-swapped). Other convenient features include power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed. USB is intended to help retire all legacy varieties of serial and parallel ports.
USB can connect computer peripherals such as mice, keyboards, printers, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives. For many of those devices,
USB has become the standard connection method. The
USB design is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. The
USB specification is at version 2.0 with revisions; Hi-Speed 2.0 rate is 480 Mbit/s (60 MB/s).

Types of
USB connectors

There are several types of
USB connectors, and some have been added as the specification has progressed. The original
USB specification detailed Standard-A and Standard-B plugs and receptacles. The first engineering change notice to the
USB 2.0 specification added Mini-B plugs and receptacles. The Mini-B, Micro-A, Micro-B, and Micro-AB connectors are used for smaller devices such as PDAs, mobile phones or digital cameras. Micro-
USB was announced by the USB-IF in January, 2007, and was intended to replace the Mini-USB plugs used in many new smart-phones and PDAs.

Cable Lengths

The maximum length of a standard
USB cable is 5.0 meters (16.4 ft). The primary reason for this limit is the maximum allowed round-trip delay of about 1500 ns. If a
USB device does not answer to host commands within the allowed time, the host considers the command to be lost.

Although a single cable is limited to 5 meters, the
USB specification permits up to five
USB hubs in a long chain of cables and hubs. Consequently the maximum possible signaling distance is 30 meters, using six 5-meter cables and five hubs. In actual use, the last hub is a more convenient endpoint since some
USB devices include built-in cables intended to directly connect to a hub, setting the maximum useful signaling distance at 25 meters.

USB compared with Firewire® IEEE-1394

USB was originally seen as a complement to IEEE 1394, which was designed as a high-speed serial bus which could efficiently interconnect peripherals such as hard disks, audio interfaces, and video equipment.
USB originally operated at a far lower data rate and used much simpler hardware, and was suitable for small peripherals such as keyboards and mice. Most significantly:

USB networks use a tiered-star topology, while IEEE 1394 networks use a repeater-based topology.

USB uses a "speak-when-spoken-to" protocol; peripherals cannot communicate with the host unless the host specifically requests communication. An IEEE 1394 device can communicate with any other node at any time, subject to network conditions.

A
USB network relies on a single host at the top of the tree to control the network. In an IEEE 1394 network, any capable node can control the network.

USB runs with a 5v power line, whereas IEEE 1394 can supply up to 30v.

USB was designed for simplicity and low cost, while IEEE 1394 was designed for high performance, particularly in time-sensitive applications such as audio and video.

The newer IEEE 1394 800 standard is twice as fast as IEEE 1394 400 and outperforms
USB 2.0 Hi-Speed both theoretically and practically.

USB is less expensive, while IEEE 1394 is more expensive to implement and requires more expensive hardware.

The Cutting Edge ~ Newest Technology

USB 3.0

A Super-Speed rate of 4.8 Gbit/s (600 MB/s),
USB 3.0 specification will be released by Intel and its partners in mid 2008 according to early reports from CNET news. Bus speeds will be 10 times faster than USB 2.0 due to the inclusion of a fiber optic link that works with traditional copper connectors. Products using the 3.0 specification are likely to arrive in 2009 or 2010.
USB 3.0 is designed to be backwards-compatible with
USB 2.0 and
USB 1.1 and employs more efficient protocols to conserve power.